PCN WG (Pre-Congestion Notification)

– a brief status update

Philip Eardley, BTTSVAREA, IETF-73 Minneapolis

18 Nov 08

http://tools.ietf.org/wg/pcn/

Main message

• The key docs are at critical stage…– Architecture (Info): about to do IETF Last Call– Baseline encoding (Std): WG Last Call very soon– Marking behaviour (Std): WG Last Call very soon– Feedback welcome asap

• … so you may want to think about how to implement and/or use it, or how it might impact on what you’re doing

What is Pre-Congestion Notification?

• Objective:– Protect the quality of service of established inelastic flows within a

single DiffServ domain,– by using flow admission and termination based on pre-congestion

information• Flow Admission (for “normal” situations) • Flow Termination (for “unusual” situations, eg failures)

• Motivation:– Admission control: there are a range of existing CAC approaches

with a range of scalability-accuracy tradeoffs• classic trade-off between flow admission ctrl (smarts) vs. generous

sizing (capacity)• PCN offers a scalable light-weight solution with strong QoS assurances

– Flow termination: typical current approach is just to wait for users to hang up

• PCN rapidly removes some flows in order to preserve QoS of the rest

BTS

MSC

PCN

V

V

PCN-boundary-node

PCN-interior-node

V

BTS

MSCPCN-boundary-node

PCN: Example scenario

• PCN-boundary-node = VoIP Trunk GW• VoIP Trunk GW handles high number of calls• All per-flow state restricted to VoIP GWs

BTS

MSC

PCN

V

V

PCN-boundary-node

PCN-interior-node

V

BTS

MSCPCN-boundary-node

PCN: Example scenario

• monitor PCN-marks (for a pair of boundary nodes)• and hence make decisions about whether to admit new flows (or terminate existing flows)

• measure current load of PCN-traffic• selectively mark packets if congestion appears imminent (hence pre-congestion)

• DiffServ forwarding• typically EF PHB

PCN Concept

• 2 levels of pre-congestion notification encoded into pkt headers• or 1 level if short of PCN codepoints

==Marking behaviour== ==PCN mechanisms== Rate of ^ PCN -traffic on | bottleneck link | (as below and also) | (as below) Drop some PCN -pkts | scheduler rate -|------------------------------------------------ (for PCN -traffic) | | Some pkts Terminate some | excess -traffic -marked admitted flows | & & | Rest of pkts Block new flows | threshold -marked | PCN -excess-rate -|------------------------------------------------ (=PCN-supportable -rate)| | All pkts Block new flows | threshold -marked | PCN-threshold -rate -|------------------------------------------------ (=PCN-admissible -rate)| | No pkts Admit new flows | PCN -marked |

Encoding• (STD) Baseline encoding

– Uses 1 DSCP • Allows re-use of other DSCPs

– Switches on PCN behaviour using ECN field– Compatible with existing tunnels (RFC 3168 & 4301)– Gives 1 level of PCN-marking

DSCP 00 10 01 11

DSCP1 Not-PCN NM(not marked)

EXP PM(PCN-

marked)

• (EXPTL) Extension encodings– Extra encoding states so can distinguish different sorts of PCN-marking

Next steps

• Complete current WG docs to RFC– Architecture; baseline encoding; marking behaviour

• Progress one or more boundary node behaviours (EXPTL)– ie methods to do flow admission & termination based

on the PCN-marked pkts– Several on the table

• Progress one or more Encoding extensions (EXPTL)– Several on the table

• Signalling reqts for PCN info (eg ingress informed about measurements made by egress)

• Spare slides

PCN motivation• flow admission ctrl (smarts) vs. generous sizing (capacity)

• classic trade-off with diseconomy of scale either way• the more hops away from admission control smarts• the more generous sizing is needed for the voice/video class

edge & interiorflow admission controlover Diffserv

edge flowadmission controlover Diffserv

Backbone provider

CustomerN/wk

CustomerN/wk AccessAccess BackhaulBackhaul CoreCore

Customerrouter

MetroNode

MSAN

Customer Access Provider

CustomerN/wk

CustomerN/wkAccessAccessBackhaulBackhaul

Customerrouter

MetroNode

MSAN

CustomerAccess Provider

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PCN with Diffserv

Marking behaviour

+ --------- + Result + ->|Threshold| ------- + | | Meter | | | + --------- + V + --------- + + - - - - -+ | + ------ + | BA | | | | | | Marked Packet =>|Classify |==>|Condition|==?================>|Marker |==> Packet Stream | | | | | | | Stream + --------- + + - - - - -+ | + ------ + | + --------- + ^ | | Exce ss | | + ->| Traffic | ------- + | Meter | Result + --------- + PCN-interior-node

Is it a PCN-pkt? (optional)

If rate of PCN-pkts > PCN-threshold-rate then mark all pkts

Write encoding in pkt

If rate of PCN-pkts > PCN-excess-rate then mark pkts above this rate